1. Field of the Invention
The present invention relates to a communication apparatus connected to a network having a transfer function.
2. Description of the Related Art
A conventional communication system is constituted by a main apparatus accommodating an outside line and an extension, and a radio communication unit (to be referred to as an extension radio communication unit hereinafter) connected to the extension. Assume that the supply of power from a commercial power supply to the main apparatus is stopped owing to a power failure or the like. In this case, if the main apparatus has a backup power supply, the main apparatus can operate as long as the backup power supply lasts.
In the above conventional communication system, however, the extension radio communication unit has no arrangement for detecting the stoppage of the main apparatus. For this reason, even if the supply of power from the backup power supply is stopped, and the operation of the main apparatus is stopped, the extension radio communication unit keeps operating in a private communication operation mode (to be referred to as a private communication mode). Consequently, the operator cannot communicate with a station outside the communication system through a public line by using the extension radio communication unit.
In addition, a station outside the communication system cannot detect that the main apparatus cannot be used. For this reason, the station cannot communicate with the communication system.